The invention relates to a method and to configurations for error masking in the transmission and/or storage of digital data, in particular in the case of application of source coding (for example speech coding using the Code Excited Linear Predictive-CELP principle). Within the scope of the invention, transmission (sending and/or receiving) is also understood as the transfer of data to/from a storage medium, that is to say also the storage of data.
Source signals or source information such as speech, sound, image and video signals virtually always include statistical redundance, that is to say redundant information. This redundancy can be greatly reduced by source coding, thus permitting efficient transmission and/or storage of the source signal. This reduction in redundancy removes, before transmission, redundant signal contents which are based on the prior knowledge of, for example, statistics of the signal shape. The bit rate of the source-coded information is also called coding rate or source bit rate. After the transmission, these components are added to the signal again during the source decoding so that no loss of quality can be detected objectively and/or subjectively.
On the other hand, it is customary in signal transmission specifically to make use of channel coding to add redundancy again, in order largely to eliminate the influence of channel faults on transmission. Additional redundant bits permit the receiver and/or decoder to detect errors and possibly also to correct them. The bit rate of the channel-coded information is also called gross bit rate.
In order to be able to transmit as efficiently as possible information, in particular speech data, image data or other useful data, by means of the limited transmission capacities of a transmission medium, in particular of an air interface, this information which is to be transmitted is therefore compressed before transmission by a source coding and protected against channel errors by a channel coding. Different methods are respectively known for these purposes. Thus, for example, in the GSM (Global System for Mobile Communication) speech can be coded by means of a full rate speech codec, a half rate speech codec or an enhanced full rate speech codec.
Within the scope of this application, a method for encoding and/or for corresponding decoding, which can also comprise a source and/or channel coding, is also denoted as a speech codec.
Residual bit errors which cannot be corrected by the channel decoding, occasionally lead to a substantial impairment of the speech reproduction. An additional method for error masking is capable of substantially improving the subjectively perceived reproduction quality.
By way of example, in the case of a frame extinction in the GSM, the last correctly received speech frame is repeated instead of the instantaneous one. After five successive faulty frames muting is performed in stages. This method is controlled by binary frame reliability information, the Bad Frame Indicator (BFI).
Soft bit speech decoding can be used for error masking by systematic expansion of the reliability information passed on to the speech decoder by the channel decoder. Methods are known in this case (Tim Fingscheidt, Peter Vary, xe2x80x9cError Concealment by Softbit Speech Decodingxe2x80x9d, ITG Fachbericht No. 139 xe2x80x9cSprachkommunikationxe2x80x9d [xe2x80x9cSpeech communicationxe2x80x9d], pages 7-10, Frankfurt a.M., 1996), in which methods of decision and estimation theory are used and are briefly explained below.
The signal-to-noise power ratio (SNR) between transmitted and received parameter values proves to be an informative quality criterion for the majority of the speech parameters determined, for example, by an CELP speech codec. Subjectively perceived speech quality and parameter SNR are thereby well correlated as a rule.
For this reason, a parameter decoder (for example a speech coder based on CELP) is sensible; it maximizes this SNR between a transmitted parameter (which is not, however, limited to speech parameters) X and the appropriately decoded parameter {circumflex over (X)} on average, or minimizes their quadratic difference, that is to say                               X          ^                =                  arg          ⁢                      xe2x80x83                    ⁢          min          ⁢                      xe2x80x83                    ⁢                      E                          x              ~                                ⁢                      {                                          (                                  X                  -                                      X                    ~                                                  )                            2                        }                                              (        1.1        )            
In order to transmit the continuous-value parameter X, it is first necessary to undertake coding by means of the bit sequence {x1, . . . xW}. This is performed by quantization, as a rule. For this purpose, the entire value range of the parameter X is decomposed into 2W intervals (or cells in the case of vector quantization) Si. Each of these cells is respectively assigned a unique transmit bit sequence xi={x1(i), . . . xW(i)}.
A channel with a binary input x and continuous-value output z is now adopted as a transmission channel. In this case, the values z observed at the output are a function of the input values x and a random process which is not initially specified in more detail. All that is presupposed for interference which acts on successive bit sequences and/or parameters is statistical independence. This channel can be completely described by the likelihood function pz|x(z1, . . . zW|X1, . . . XW).
The expectation in (1.1) is therefore determined by two random processes: by the parameter-generating process X and the observed received values z, that is to say                               E          X                ,                                            {                              z1                ,                …                ⁢                                  xe2x80x83                                ,                zW                            }                        ⁢                          xe2x80x83                        ⁢                          {                                                [                                      X                    -                                                                  X                        ~                                            ⁡                                              (                                                                              z                            1                                                    ,                          ,                                                      z                            W                                                                          )                                                                              ]                                2                            }                                =                                    ∫                                                z                  1                                ,                …                ⁢                                  xe2x80x83                                ,                                  z                  W                                                      ⁢                                          ∫                X                            ⁢                                                                    [                                          X                      -                                                                        X                          ~                                                ⁡                                                  (                                                                                    z                              1                                                        ,                            …                            ⁢                                                          xe2x80x83                                                        ,                                                          z                              W                                                                                )                                                                                      ]                                    2                                ⁢                                                      p                                          x                      ,                                              z                        1                                            ,                      …                      ⁢                                              xe2x80x83                                            ,                                              z                        W                                                                              ⁡                                      (                                          X                      ,                                              z                        1                                            ,                      …                      ⁢                                              xe2x80x83                                            ,                                              z                        W                                                              )                                                  ⁢                                  ⅆ                  X                                ⁢                                  ⅆ                                      z                    1                                                  ⁢                …                ⁢                                  xe2x80x83                                ⁢                                                      ⅆ                                          z                      W                                                        .                                                                                        (        1.2        )            
Since the integrand is always positive, the expectation can be minimized by minimizing the inner integral with respect to {tilde over (X)} for each possible reception sequence {z1, . . . ,zW}. The result is the formula of the Mean Square (MS) estimator                                           X            ~                    opt                =                              ∫            X                    ⁢                                    X              ·                                                p                                                            x                      |                                              z                        1                                                              ,                    …                    ⁢                                          xe2x80x83                                        ,                                          z                      W                                                                      ⁡                                  (                                                            X                      |                                              z                        1                                                              ,                    …                    ⁢                                          xe2x80x83                                        ,                                          z                      W                                                        )                                                      ⁢                          ⅆ              X                                                          (        1.3        )            
Taking account of the quantization at the transmitting end, this further yields                                           X            ~                    opt                =                              ∑            1                    ⁢                                    ∫                              X                ∈                                  S                  i                                                      ⁢                                          X                ·                                                                                                    p                                                                              z                            1                                                    ,                          …                          ⁢                                                      xe2x80x83                                                    ,                                                                                    z                              W                                                        |                            x                                                                                              ⁡                                              (                                                                              z                            1                                                    ,                          …                          ⁢                                                      xe2x80x83                                                    ,                                                                                    z                              W                                                        |                            x                                                                          )                                                              ·                                                                  p                        x                                            ⁡                                              (                        X                        )                                                                                                                        p                                                                        z                          1                                                ,                        …                        ⁢                                                  xe2x80x83                                                ,                                                  z                          W                                                                                      ⁡                                          (                                                                        z                          1                                                ,                        …                        ⁢                                                  xe2x80x83                                                ,                                                  z                          W                                                                    )                                                                                  ⁢                                                ⅆ                  X                                .                                                                        (        1.4        )            
Since the same bit sequence xi is transmitted for all Xxcex5Si, the conditional probability in the numerator is a constant with respect to the integration, and it follows that                                                                                           X                  ~                                ⁢                                  xe2x80x83                                ⁢                opt                            =                                                ∑                  i                                ⁢                                                                                                    p                                                                              z                            1                                                    ,                          …                          ⁢                                                      xe2x80x83                                                    ,                                                                                    z                              W                                                        |                            x                                                                                              ⁡                                              (                                                                              z                            1                                                    ,                          …                          ⁢                                                      xe2x80x83                                                    ,                                                                                    z                              W                                                        |                                                          x                              i                                                                                                      )                                                                                                            p                                                                              z                            1                                                    ,                          …                          ⁢                                                      xe2x80x83                                                    ,                                                      z                            W                                                                                              ⁡                                              (                                                                              z                            1                                                    ,                          …                          ⁢                                                      xe2x80x83                                                    ,                                                      z                            W                                                                          )                                                                              ⁢                                                            ∫                                              X                        ∈                                                  S                          i                                                                                      ⁢                                                                  X                        ·                                                                              p                            x                                                    ⁡                                                      (                            X                            )                                                                                              ⁢                                              ⅆ                        X                                                                                                                                                                    =                                                ∑                  i                                ⁢                                                                                                    p                                                                              z                            1                                                    ,                          …                          ⁢                                                      xe2x80x83                                                    ,                                                                                    z                              W                                                        |                            x                                                                                              ⁡                                              (                                                                              z                            1                                                    ,                          …                          ⁢                                                      xe2x80x83                                                    ,                                                                                    z                              W                                                        |                                                          x                              i                                                                                                      )                                                                                                            p                                                                              z                            1                                                    ,                          …                          ⁢                                                      xe2x80x83                                                    ,                                                      z                            W                                                                                              ⁡                                              (                                                                              z                            1                                                    ,                          …                          ⁢                                                      xe2x80x83                                                    ,                                                      z                            W                                                                          )                                                                              ·                                      E                    ⁡                                          (                                              X                        |                                                  x                          i                                                                    )                                                        ·                                      Pr                    ⁡                                          (                                              x                        i                                            )                                                                                                                                              =                                                ∑                  i                                ⁢                                                      E                    ⁡                                          (                                              X                        |                                                  x                          i                                                                    )                                                        ·                                      Pr                    ⁡                                          (                                                                                                    x                            i                                                    |                                                      z                            1                                                                          ,                        …                        ⁢                                                  xe2x80x83                                                ,                                                  z                          W                                                                    )                                                                                                                              (        1.5        )            
If the parameter-generating process X is not devoid of memory, there are additional statistical connections between successive parameter values and/or bit sequences x.
An analogous derivation then follows taking account of the time index n                                                         X              ~                        opt                    ⁡                      (            n            )                          =                              ∑            i                    ⁢                                    E              ⁡                              (                                  X                  |                                      x                    i                                                  )                                      ·                                          Pr                ⁡                                  (                                                                                    x                        i                                            |                                                                        z                          1                                                ⁡                                                  (                          n                          )                                                                                      ,                    …                    ⁢                                          xe2x80x83                                        ,                                                                  z                        W                                            ⁡                                              (                        n                        )                                                              ,                    …                    ⁢                                          xe2x80x83                                        ,                                                                  z                        1                                            ⁡                                              (                        0                        )                                                              ,                    …                    ⁢                                          xe2x80x83                                        ,                                                                  z                        W                                            ⁡                                              (                        0                        )                                                                              )                                            .                                                          (        1.6        )            
It is shown below how the a posteriori probabilities can be determined in (1.5) and (1.6):
It may be assumed by way of simplification that the transmission channel located between a source coder and a source decoder and comprising a channel coder, modulator, physical channel, demodulator, equalizer and channel decoder can be considered, as shown in FIG. 3, as a faulty channel, devoid of memory at the bit level, with a binary input x and continuous output z. In this case, the source coder is illustrated in a simplified fashion by a parameter coder PC, and the source decoder is illustrated in a simplified fashion by an estimator S. This channel is completely described by the likelihood function pz|y(zk|yk) and/or by the L values L(zk|xk). Initially, the a posteriori probability is to be specified for white parameter sources in accordance with (1.5). The Bayes formula is applied for this purpose:                               Pr          ⁡                      (                                                            x                  i                                |                                  z                  1                                            ,              …              ⁢                              xe2x80x83                            ,                              z                W                                      )                          =                                                            p                                                      z                    1                                    ,                  …                  ⁢                                      xe2x80x83                                    ,                                                            z                      W                                        |                    x                                                              ⁡                              (                                                      z                    1                                    ,                  …                  ⁢                                      xe2x80x83                                    ,                                                            z                      W                                        |                                          x                      i                                                                      )                                                                    ∑                j                            ⁢                                                                    p                                                                  z                        1                                            ,                      …                      ⁢                                              xe2x80x83                                            ,                                                                        z                          W                                                |                        x                                                                              ⁡                                      (                                                                  z                        1                                            ,                      …                      ⁢                                              xe2x80x83                                            ,                                                                        z                          W                                                |                                                  x                          j                                                                                      )                                                  ·                                  Pr                  ⁡                                      (                                          x                      j                                        )                                                                                ·                      Pr            ⁡                          (                              x                i                            )                                                          (        1.7        )            
All the variables are now known in this: the likelihood function pz1, . . . ,zW|x(z1, . . . ,zW|xi) is given by the channel characteristics, and Pr(xi) is the a priori probability that the bit sequence xi is being transmitted. These probabilities are measured in advance for all i with the aid of representative signal data (for example speech samples in the case of speech coding).
The likelihood function can be written as a product since a channel which is devoid of memory at the bit level was presupposed                                           P                                          z                1                            ,              …              ⁢                              xe2x80x83                            ,                                                z                  W                                |                x                                              ⁡                      (                                          z                1                            ,              …              ⁢                              xe2x80x83                            ,                                                z                  W                                |                                  x                  i                                                      )                          =                              ∏                          k              =              1                        W                    ⁢                      xe2x80x83                    ⁢                                                    p                                  z                  |                  x                                            ⁡                              (                                                      z                    k                                    |                                                            x                      k                                        ⁡                                          (                      i                      )                                                                      )                                      .                                              (        1.8        )            
If the source decoder is provided with reliability information on the parameter X in the form of an L value sequence {L(z1|x1), . . . ,L(zW|xW)}, it is possible to calculate (1.8) efficiently in the logarithmic region. The following is yielded                                           p                                          z                1                            ,              …              ⁢                              xe2x80x83                            ,                                                z                  W                                |                x                                              ⁡                      (                                          z                1                            ,              …              ⁢                              xe2x80x83                            ,                                                z                  W                                |                                  x                  i                                                      )                          =                  C          ·                                    exp              ⁡                              (                                                      ∑                                          k                      ∈                                              {                                                                              k                            |                                                                                          x                                k                                                            ⁡                                                              (                                i                                )                                                                                                              =                                                      +                            1                                                                          }                                                                              ⁢                                      L                    ⁡                                          (                                                                        z                          k                                                |                                                  x                          k                                                                    )                                                                      )                                      .                                              (        1.9        )            
In this case, C is a constant which can be determined by the normalizing condition                                           ∑            i                    ⁢                      Pr            ⁡                          (                                                                    x                    i                                    |                                      z                    1                                                  ,                …                ⁢                                  xe2x80x83                                ,                                  z                  W                                            )                                      =        1                            (        1.10        )            
If the parameter source X is not white, that is to say there is a statistical dependence between successive output values, the source can be modeled by a Markov chain. Only the case of a first order Markov chain is to be considered here. It holds in this case with zn={z1(n), . . . ,zw(n)}                              Pr          ⁡                      (                                                                                x                    i                                    ⁡                                      (                    n                    )                                                  |                                  z                  n                                            ,                              z                                  n                  -                  1                                            ,              …              ⁢                              xe2x80x83                            ,                              z                0                                      )                          =                                            p              ⁡                              (                                                      z                    n                                    ,                  …                  ⁢                                      xe2x80x83                                    ,                                                            z                      0                                        |                                                                  x                        i                                            ⁡                                              (                        n                        )                                                                                            )                                                    p              ⁡                              (                                                      z                    n                                    ,                  …                  ⁢                                      xe2x80x83                                    ,                                      z                    0                                                  )                                              =                                                    P                ⁡                                  (                                                            z                      n                                        |                                                                  x                        i                                            ⁡                                              (                        n                        )                                                                              )                                            ·                                                                    ∑                    j                                    ⁢                                      p                    ⁡                                          (                                                                                                    x                            i                                                    ⁡                                                      (                            n                            )                                                                          ,                                                                              x                            j                                                    ⁡                                                      (                                                          n                              -                              1                                                        )                                                                          ,                                                  z                                                      n                            -                            1                                                                          ,                        …                        ⁢                                                  xe2x80x83                                                ,                                                  z                          0                                                                    )                                                                                        p                  ⁡                                      (                                                                  z                        n                                            ,                      …                      ⁢                                              xe2x80x83                                            ,                                              z                        0                                                              )                                                                        ⁢                          xe2x80x83                        ⁢                                                            p                  ⁡                                      (                                                                  z                                                  n                          -                          1                                                                    ,                      …                      ⁢                                              xe2x80x83                                            ,                                              z                        0                                                              )                                                                                        p                    ⁡                                          (                                                                        z                          n                                                ,                        …                        ⁢                                                  xe2x80x83                                                ,                                                  z                          0                                                                    )                                                                            ⏟                                          const                      .                                                                                  ·                              "AutoLeftMatch"                                  xe2x80x83                                ⁢                                                      p                    (                                          xe2x80x83                                        ⁢                                                                  z                        n                                            |                                                                        x                          i                                                ⁡                                                  (                          n                          )                                                                                      )                                    ·                                                            ∑                      j                                        ⁢                                                                  Pr                        ⁡                                                  (                                                                                                                    x                                i                                                            ⁡                                                              (                                n                                )                                                                                      |                                                                                          x                                j                                                            ⁡                                                              (                                                                  n                                  -                                  1                                                                )                                                                                                              )                                                                    ·                                              Pr                        ⁡                                                  (                                                                                                                                                      x                                  i                                                                ⁡                                                                  (                                                                      n                                    -                                    1                                                                    )                                                                                            |                                                              z                                                                  n                                  -                                  1                                                                                                                      ,                            …                            ⁢                                                          xe2x80x83                                                        ,                                                          z                              0                                                                                )                                                                                                                                                                            (        1.11        )            
The indices of the distribution density functions have been dispensed with in the derivation for reasons of clarity. The fraction in (1.11) is a constant with respect to xi(n), and can be determined with the aid of the normalizing condition (1.10). The transitional probabilities (Pr(xi(n)|xj(nxe2x88x921)) of the Markov chain must be measured just like the a priori probabilities on a long output sequence of the parameter source.
An expansion of this recursion formula to a Markov chain of higher order is certainly directly possible, but the increase thereby caused in computational complexity and required storage space is not, as a rule, acceptably related to the increase in accuracy of estimation.
It is accordingly an object of the invention to provide a method and a configuration for error masking which permit a high-quality reconstruction of source signals from signal data transmitted over a transmission channel affected by error, and which overcomes the above-mentioned disadvantageous of the prior art methods and configurations of this general type.
With the foregoing and other objects in view there is provided, in accordance with the invention a method for error masking that can be used when transmitting parameter values. The method includes steps of: converting parameter values into a binary representation; at a transmitting end, adding first redundant information to at least some of the parameter values by precoding the binary representation of the at least some of the parameter values with a linear block code, and performing the preceding before transmitting the binary representation of the at least some of the parameter values over a faulty channel; and using the first redundant information at a receiving end to estimate the parameter values rather than using the first redundant information for error detection.
The invention is therefore based on the idea that redundancy which is still present in the transmitted signal after the source coding and added explicitly in the form of channel coding is used at the receiving end not for error detection or correction, but is utilized for an estimate which is optimum for the purpose of a measure of quality.
It is therefore possible to carry out the error masking with a higher quality.
In accordance with an added feature of the invention, the method includes steps of: at the transmitting end, performing at least a source coding to obtain the parameter values; adding second redundant information to the precoded binary representation of the at least some of the parameter values; and after the binary representation of the at least some of the parameter values have been transmitted over the faulty channel to a receiving end, using the second redundant information, at the receiving end, for a process selected from the group consisting of error detection and error correction.
In accordance with an additional feature of the invention, the second redundant information is added for channel coding.
In accordance with another feature of the invention, the first redundant information is used at the receiving end as a posteriori information to estimate the parameter values.
In accordance with a further feature of the invention, a soft bit speech decoding method is used to estimate the parameter values.
With the foregoing and other objects in view there is also provided, in accordance with the invention a transmitting device for transmitting parameter values. The transmitting device includes a processor unit configured for: converting parameter values into a binary representation; adding first redundant information to at least some of the parameter values by preceding the binary representation of the at least some of the parameter values with a linear block code, and performing the preceding before transmitting the binary representation of the at least some of the parameter values over a faulty channel; and adding the first redundant information to the at least some of the parameter values in a manner such that at a receiving end, the first redundant information can be used for estimating the parameter values rather than for error detection.
With the foregoing and other objects in view there is provided, in accordance with the invention a receiving device for receiving parameter values which have been converted into a binary representation at a transmitting end before being transmitted over a faulty channel. First redundant information has been added to the binary representation of the parameter values by preceding with a linear block code before the binary representation of the parameter values have been transmitted over the faulty channel. The receiving device includes a processor unit configured for using the first redundant information to estimate the parameter values rather than for using the first redundant information for error detection.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and configuration for error masking, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.